Abstract

Scanning tunnelling microscopy (STM) has been used to study the adsorption structures formed by atomic nitrogen on Cu(1 0 0), produced by low energy nitrogen ion bombardment and annealing. Atomic resolution imaging under varying tip conditions shows that in the c(2 x 2) phase STM appears to always image as asperities the Cu atoms, and not the N atoms as has been previously supposed. These images also indicate that in this phase the N induces a novel large-amplitude symmetry-lowering rumpling reconstruction of the outermost Cu layer. This conclusion allows one to understand many aspects of the mesoscopic c(2 x 2) island structures observed in this system and also reported in earlier STM investigations, and specifically the systematic behaviour of inter-island boundary widths which provide an indirect indication of the symmetry reduction. The possible role of adsorbate-induced surface stress changes and strain perpendicular to the surface at both local and longer ranges in the formation and self-organisation of the islands is discussed. Evidence for local strain and information on the anisotropy of step energies in the saturation coverage 'trench' phase is also discussed. (C) 2001 Elsevier Science B.V. All rights reserved.